This invention relates to a system and method for determining the location of a stationary or mobile transmitting unit such as, for example, mobile police units and emergency vehicles. For example, it is desirable to ascertain the location of police and emergency vehicles in order to be able to provide help both rapidly and effectively. It is also desirable to be able to locate vehicles carrying valuable documents or cargo on a time scheduled route in order to ascertain their actual position in reference to their scheduled position.
Furthermore, it is desirable for a main communication site, such as a taxi or delivery vehicle dispatcher, to be able to monitor the movement or location of each of a plurality of mobile units. Still further, it may be desirable to locate persons, such as policemen and watchmen, not associated with vehicles.
Typically, such units, both persons and vehicles, are equipped with radios that communicate with a primary location, such as a dispatcher. In order for the dispatcher to know the location of a unit, it is generally necessary for the unit to broadcast the location. However, there are systems which transmit a unique tone over a transmitter to receivers stationed in a local area for determining the transmitter's location.
Prior art systems for locating a vehicle (such as are described in U.S. Pat. Nos. 3,848,254 and 3,680,121) utilize a method in which the vehicle transmits a steady tone which is received by at least three spaced radio receiving stations. In the system described in U.S. Pat. No. 3,848,254, the signal received at each station is retransmitted to a central site where the difference in time of arrival of the signal at each station is determined by phase comparison. These systems thus require generation of a fixed frequency signal and phase shift comparison of received signals at a central site for determining the location of the transmitting vehicle.
In another system, described in U.S. Pat. No. 3,419,865, a calibrated timing source is used for generating pulses at predetermined intervals. Each vehicle using this system is equipped with a pulse transmitter and a clock synchronized to a common clock system to actuate the vehicle transmitter during a unique time slot assigned to that vehicle. Multiple receivers each determine the time difference between the time of arrival of a signal from a vehicle and the time of generation of the clock pulse which initiated the vehicle signal. The time differences from each receiver are transmitted to a central site where a computer determines the location of the vehicle by solving simultaneous hyperbolic relationships. This system thus requires synchronization of each vehicle transmitter to a system clock and production of a pulse signal at preassigned time intervals.